This invention describes a sealed enclosure surrounding an electronic device, a method for packaging an electronic device and a method of maintaining a dry atmosphere in a container type sealed electronic package by incorporating a coating or adhesive with desiccant properties.
Modern container type packagings used for electronic devices, such as integrated microcircuits (silicon chips or dies), hybrid microcircuits and surface mounted devices used as transistors, diodes, resistors, capacitors, transducers and such, are designed both to protect the sensitive components and circuits mechanically and environmentally, and to provide a functional interface to macroscopic application, such as to a printed circuit board.
There has been continuing concern regarding the presence of water vapor in hermetically sealed semiconductor devices. This concern is based upon both theoretically possible failure modes and actual observation of failure caused by corrosion due to the presence of moisture.
Hermetic microelectronic devices used in military, space and other applications requiring high reliability have an upper limit of 5,000 parts per million (ppm, by volume), of water vapor content at the time of fabrication. Package leak rate is limited to 10.sup.-8 atm -cc/sec. maximum to prevent leakage of a significant amount moist ambient air into the packaging during the device's useful lifetime. In spite of extreme precaution, it is very difficult to manufacture a hermetic packaging for microelectronic devices with low water vapor content and to maintain it during its useful lifetime. There are various, channels by which water vapor finds its way to the inside of the enclosure:
1. The various seals in a packaging are usually not perfect and exhibit a small but yet sufficient leakage to allow ambient air, containing moisture, inside the enclosure. PA0 2. Many epoxies used to bond dies and substrates outgas moisture in the packaging with time. PA0 3. The packaging material itself outgasses a certain amount of moisture. Prebaking prior to sealing may not liberate all the absorbed moisture. PA0 4. The sealing atmosphere may be contaminated with moisture. PA0 5. The testing process for leakage rate may itself introduce moisture inside the package if not preformed with extreme care.
The prior art has attempted to solve the moisture problem by the application of a moisture barrier coating, also referred to as passivating layer, on the microcircuit. Such products and processes include total passivation with silicone compounds and surface passivation with silicon oxide, silicon nitride and plasma deposition of polymerized hexamethyldisilazane. U.S. Pat. Nos. 4,163,072, 4,059,708 and 4,628,006, issued, repeatedly, to Soos, Heiss, Jr. et al. and Schnessler are illustrative of such methods. However, for a number of practical reasons these techniques were found to be less than adequate.